1. Field of the Invention
The present invention relates to methods for producing elastomer composites and composites produced by such methods.
2. Description of the Related Art
Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black, for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, a premixture of filler, elastomer and various optional additives, such as extender oil. Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in structure, which describes the size and complexity of aggregates of carbon black formed by the fusion of primary carbon black particles to one another. Numerous products of commercial significance are formed of such elastomeric compositions of carbon black particulate filler dispersed in natural rubber. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, sidewalls, wire skim and carcass. Other products include, for example, engine mount bushings, conveyor belts, windshield wipers and the like.
Good dispersion of carbon black in natural rubber compounds has been recognized for some time as one of the most important objectives for achieving good quality and consistent product performance, and considerable effort has been devoted to the development of procedures for assessing dispersion quality in rubber. The mixing operations have a direct impact on mixing efficiency and on macro-dispersion. In general, better carbon black macro-dispersion is achieved in a dry-mixed masterbatch by longer mixing and by more intensive mixing. Unfortunately, however, achieving better macro-dispersion by longer, more intensive mixing degrades the elastomer into which the carbon black is being dispersed. This is especially problematic in the case of natural rubber, which is highly susceptible to mechanical/thermal degradation. Longer and more intensive mixing, using known mixing techniques and apparatus, such as an internal mixer, reduces the molecular weight of the natural rubber masterbatch-composition. Thus, improved macro-dispersion of carbon black in natural rubber is known to be achieved with a corresponding, generally undesirable reduction in the molecular weight of the rubber.
In addition to dry mixing techniques, it is known to continuously feed latex and a carbon black slurry to an agitated coagulation tank. Such “wet” techniques are often used with synthetic elastomer, such as styrene butadiene rubber (SBR). The coagulation tank contains a coagulant such as salt or an aqueous acid solution typically having a pH of about 2.5 to 4. The latex and carbon black slurry are mixed and coagulated in the coagulation tank into small beads (typically a few millimeters in diameter) referred to as wet crumb. The crumb and acid effluent are separated, typically by means of a vibrating shaker screen or the like. The crumb is then dumped into a second agitated tank where it is washed to achieve a neutral or near neutral pH. Thereafter the crumb is subjected to additional vibrating screen and drying steps and the like. Variations on this method have been suggested for the coagulation of natural and synthetic elastomers. In commonly owned U.S. Pat. No. 4,029,633 to Hagopian et al., a continuous process for the preparation of elastomer masterbatch is described. An aqueous slurry of carbon black is prepared and mixed with a natural or synthetic elastomer latex. This mixture undergoes a so-called creaming operation, optionally using any of various known creaming agents. Following the creaming of the carbon black/latex mixture, it is subjected to a coagulation step. Specifically, the creamed carbon black/latex mixture is introduced as a single coherent stream into the core of a stream of coagulating liquor. The solid stream of creamed carbon black/latex mixture is said to undergo shearing and atomizing by the stream of coagulating liquor prior to coagulation, being then passed to a suitable reaction zone for completion of the coagulation. Following the coagulation step, the crumb is separated from the waste product “serum,” washed, and dried. A somewhat similar process is described in U.S. Pat. No. 3,048,559 to Heller, et al. An aqueous slurry of carbon black is continuously blended with a stream of natural or synthetic elastomer or latex. The two streams are mixed under conditions described as involving violent hydraulic turbulence and impact. As in the case of the Hagopian patent mentioned above, the combined stream of carbon black slurry and elastomer latex is subsequently coagulated by the addition of an acid or salt coagulant solution.
It is well known that mastication of dry masterbatch (e.g., produced by a dry mix process or by a wet masterbatch process, followed by drying) may be employed to reduce Mooney viscosity and improve processability while incorporating additives such as oils, antioxidants, and zinc oxide. Vulcanizing agents may be added as well or may be added in a second mastication step. However, the mixing may need to be done at lower temperatures (e.g., below 125° C.) to prevent precure or scorch. In addition, overmixing may be detrimental to viscoelastic properties and may increase flocculation during storage, which can increase storage hardening and further degrade rubber performance (Wang, et al., KGK Kauschuk Gummi Kunststoffe, Vol. 7-8, 2002, pp. 388-396).
An alternative mixing method is disclosed by commonly owned U.S. Pat. Nos. 6,048,923 and 6,929,783, which disclose a wet masterbatch process in which separate streams of a carbon black slurry and an elastomer latex are combined under conditions where the elastomer latex coagulates without the use of added coagulants. The masterbatch is dewatered to about 15% to 25% water content and then passed through a continuous compounder and, optionally, an open mill. The final product has a moisture content of less than about 1% and may have a temperature of about 100° C. after emerging from the open mill. The operating parameters of the continuous compounder, e.g., throughput rate, rotor speed, discharge orifice size and temperature, and processing chamber temperature, may be optimized to control Mooney viscosity, moisture content, molecular weight, and bound rubber. The desired values of these properties depend on the intended use of the product masterbatch. For example, the Mooney viscosity may be reduced to facilitate further processing of the masterbatch into vulcanized rubber.
U.S. Pat. No. 6,841,606 (“the '606 patent”) describes a wet masterbatch process and suggests drying the coagulum by applying a mechanical shearing force to improve the dispersion of the filler. The resulting rubber is disclosed to have good processability and reinforcement and reduce fuel consumption. In the examples, a latex masterbatch having about 50 phr of carbon black or silica was brought to about 40% moisture content and dried thermally at 120° C. or through use of a 1 meter long twin-screw kneading extruder maintained at the same temperature. Use of the drier and the extruder resulted in products having approximately equivalent tensile strength and abrasion resistance.
PCT Publication WO 2006/068078 discloses a method of making wet natural rubber masterbatch having particular grades of carbon black defined by nitrogen surface area, 24M4 DBP, and particle size distribution. As in the '606 patent, the examples describe the production of masterbatch having 50 phr of carbon black, dehydrated to about 40% moisture content, and passed through a 1 meter long twin-screw kneading extruder maintained at 120° C.
PCT Publication WO 2006/080852 discloses a wet natural rubber masterbatch including a carbon black having specific surface area, structure, and tint. As in the '606 patent, the examples describe the production of masterbatch having 50 phr of carbon black, dehydrated to about 40% moisture content, and passed through a 1 meter long twin-screw kneading extruder maintained at 120° C.
Japanese Patent Publication No. 2006-265311 discloses a wire skim composition produced using a wet rubber masterbatch having a carbon black with specific surface area or structure. As in the '606 patent, the examples describe the production of masterbatch having 50 phr of carbon black, dehydrated to about 40% moisture content, and passed through a 1 meter long twin-screw kneading extruder maintained at 120° C.
However, as carbon black loading increases, it is difficult to achieve good dispersion of the carbon black at either macroscopic (e.g., 10 micron or greater) or microscopic levels and the quality of the masterbatch at a macroscopic level, with the consequence that the resulting rubber does not exhibit desired properties. Furthermore, masterbatches produced with carbon blacks having higher structure and surface area values are more viscous, making them more difficult to compound.